A persistent challenge in the clinical management of breast cancer is why certain patients with ER+ tumors that express estrogen receptor-1 (ER1) do not benefit from hormonal therapies. ER1 proteostasis is fundamental process by which cells control sensitivity and dependence on hormones through the regulation of receptor levels. While disruptions of ER1 proteostasis result in pathological gains and losses of receptor function that bypass control by estrogens and antiestrogens, the signals and mechanisms that control ER1 proteostasis and their impact on therapeutic response are poorly understood. By increasing our understanding of ER1 proteostasis in breast cancer, the proposed studies seek to improve our ability to control and predict response to hormone-based therapies in patients with ER+ disease. Our published and preliminary studies shed light on three new areas regulating ER1 proteostasis in breast cancer cells, including new mechanistic insight controlling ER1 protein stability and function in cancer cells, contributions of the tumor microenvironment, and a new application to rigorously quantify ER1 protein levels by immunohistochemistry in tumor samples. These studies support the hypothesis that both cell-intrinsic and extrinsic factors control ER1 protein in breast tumor cells thereby contributing to therapeutic outcomes. In Aim 1, we focus on intrinsic mechanisms governing ER1 protein stability with emphasis on newly discovered regulation by the peptidyl prolyl isomerase, Pin1. In Aim 2, we apply innovative bioengineering technologies to the study of ER1 protein regulation in breast cancer cells by primary patient fibroblasts. Finally, in Aim 3, we test predictions from our molecular and cellular analysis in the complex environment of patient tumor samples using AQUA technology to establish the relationship of Pin1 and ER1 levels to therapy outcomes. This research is expected to considerably expand our knowledge of the mechanisms controlling ER1 protein and thereby enhance future strategies to extend the benefit of hormonal therapies to a greater number of women.